The present invention relates to electrolyte circulation type secondary batteries, particularly electrolyte circulation type metal-halogen secondary batteries. More particularly, the invention relates to a metal-bromine secondary battery which employs a complexing agent such that the complexing agent combines with the bromine molecules in the anode electrolyte so that the bromine molecules separated from the electrolyte form a bromine complex compound in an oily form and precipitate in the lower part of the anode electrolyte storage tank.
The electrolyte circulation type metal-halogen secondary batteries known in the art are basically constructed as shown in FIG. 1 of the accompanying drawing. In the Figure, numeral 1 designates a cathode, 2 a cathode chamber, 3 an anode, 4 an anode chamber, 5 a separator arranged substantially midway between the cathode 1 and the anode 3, 6 a cathode electrolyte storage tank, 7 an anode electrolyte storage tank, 8 an anode electrolyte including a bromine complex compound circulation means for putting the cathode chamber 2 in fluid communication with the cathode electrolyte storage tank 6 and for putting the anode chamber 4 in fluid communication with the anode electrolyte storage tank 7, which includes 9 electrolyte circulation pumps, 10 valves, and 11 a bypass for circulating the anode electrolyte containing no bromine complex compound.
While the metal used may be zinc, cobalt, cadmium or copper, if zinc is selected, for example, during charging a reaction of Zn.sup.++ +2e.sup.- .fwdarw.Zn takes place in the cathode chamber and a reaction of 2Br.sup.- .fwdarw.Br.sub.2 +2e.sup.- takes place in the anode chamber. During discharging, the reverse reactions proceed.
Note that the cathode electrolyte consists of an aqueous solution of ZnX.sub.2 (where X is a halogen atom) and the anode electrolyte consists of an aqueous solution of ZnBr.sub.2, Br.sub.2 and a bromine complexing agent.
The separator for dividing the two electrolyte chambers generally comprises a perforated membrane of polyolefin type or an ion exchange membrane.
When this type of secondary battery is charged, the bromine deposits onto the anode and the metal deposits onto the cathode. While the metal deposited onto the cathode is electrodeposited on the electrode plate, the bromine deposited onto the anode is dissolved into the electrolyte and circulated along with the electrolyte.
This is a disadvantage. During the operation of the battery, a part of the deposited bromine passes through the separator and reaches the metal side of the electrode pair thus causing a self-discharge threat. Also, the bromine itself is a highly corrosive substance, and, thus, limitations are imposed on the selection of materials for forming the battery.
In view of these restrictions, no electrolyte circulation type metal-halogen secondary battery has been put into practical use.